Sunlight Creates Quantum Entanglement Once Thought to Require Lasers

“Quantum entanglement is crucial for applications such as secure communication, ultra-precise sensing and high-performance computation,” said Cheng Li, a recent graduate of the University of Ottawa in Canada. “Our research shows that abundant natural light sources can generate quantum entanglement, creating new opportunities for energy-efficient quantum technologies.”

The findings, published in Optica, the high-impact research journal of Optica Publishing Group, demonstrate that sunlight can produce quantum entanglement comparable to laser-based methods when differences in the incoming light’s bandwidth are considered. The study combines theoretical research from Robert Boyd’s group at the University of Ottawa with an advanced solar concentrator developed by Hanieh Fattahi’s team at the Max Planck Institute for the Science of Light (MPL) in Germany.

“This technology could eventually allow satellites to generate secure encryption keys using the sunlight that is already abundant in space,” said Li, the paper’s first author. “That could reduce the need for onboard lasers and associated hardware. Sunlight-based entanglement generation may also provide an important component for scaling quantum computing without significantly increasing its energy demands.”

Rethinking How Quantum Entanglement Is Produced

For decades, scientists assumed that producing the strong correlations required for photon entanglement depended on coherent light. In coherent light, waves remain synchronized, with their peaks and valleys following a predictable pattern. Lasers are widely used because they produce highly coherent light concentrated around a narrow range of colors.

Previous research by Boyd’s team challenged this conventional view. The researchers predicted and experimentally demonstrated that incoherent light could also produce quantum entanglement. In those experiments, an LED served as the incoherent light source for generating polarization-entangled photons.

These results revealed that light can be disordered in one property, such as its direction of travel, while still producing photons that are entangled through another property, such as polarization.

The latest study advances that concept by replacing the LED with sunlight. This created a far more demanding test because sunlight travels in many directions and contains a broad spectrum of colors.

How Sunlight Generates Entangled Photons

The researchers generated entangled photons using spontaneous parametric down-conversion (SPDC), a well-established nonlinear optical process. In SPDC, a pump beam enters a nonlinear crystal, causing individual photons to split into pairs that can become quantum entangled.

Instead of using a conventional laser pump, the researchers directed sunlight into the system. The sunlight was strongly polarized but remained highly incoherent across both space and time. In other words, its overall electric field oscillated in the same direction even though it contained different colors of light traveling along numerous paths.

“We designed the experiment so that variations caused by different colors and propagation directions would not affect the photons’ polarization,” Li explained. “Our theory predicted that if the entanglement exists only in polarization, it should depend on the pump’s orderliness in its oscillation direction, rather than on its direction of travel or color. This made it possible to generate high-quality polarization entanglement from sunlight that was highly incoherent in space and time.”

Concentrating enough sunlight onto the extremely small nonlinear crystal presented another major challenge. The crystal is only about one millimeter in size, making conventional sunlight-collection methods unsuitable.

Fattahi’s team at MPL solved this problem by developing an all-glass solar concentrator. The cone-shaped device uses a Fresnel lens roughly the size of a household window to collect sunlight and channel it into an optical fiber about as wide as a human hair. The concentrated sunlight can then be delivered to the tiny nonlinear crystal used to create entangled photons.

Sunlight Produces High-Quality Quantum Entanglement

The researchers evaluated their theoretical predictions and solar concentrator during an outdoor experiment at MPL. Using quantum state tomography, they analyzed the resulting quantum state and found that the sunlight-generated entanglement was approximately 94% similar to a perfectly entangled state.

The experiment also showed that the photons produced correlations that violated Bell’s inequality. This is a significant result because Bell inequality violations cannot be explained by classical physics and serve as evidence of genuine quantum entanglement.

Following the successful proof-of-principle demonstration, the researchers are developing a system that could eventually operate outside a laboratory environment. Their next goals include increasing the brightness of the entangled-photon source and improving the overall quality of the quantum entanglement.

Although the experiment used SPDC, the researchers believe the same principle could be applied to other nonlinear optical processes, including four-wave mixing. Applying sunlight-based entanglement generation to additional techniques could expand its potential across quantum photonics, satellite communications and other emerging technologies.

Turning Scientific Doubt Into a Successful Demonstration

The achievement also addressed skepticism about whether sunlight could realistically drive a nonlinear optical process capable of producing detectable entangled photons.

“From the beginning of this project, our idea faced repeated doubt and resistance,” Li said. “Some leading researchers questioned whether it would be possible to detect any photons from sunlight-driven nonlinear optical processes, let alone entangled photons. However, we trusted our calculations, continued refining the experimental system and ultimately demonstrated that it could be done.”